On the journey itself, near the end, he comes across a man, sitting in a chair in one of the buildings. From this, and all of the other clues that he picks up on his journey, John learns that the gods "were men -- they went a dark road, but they were men". He realizes how advanced they were, but also how they destroyed each other. This is significant information, considering who they have been revering for so long were just men, like them.
The true key insight arrives when he gets back from his journey. His dad asks him to tell him everything,and once he does, his father states, "Truth is a hard deer to hunt. If you eat too much truth at once, you may die of the truth." He is basically saying that the people have been living a certain way, and believe certain things, and the information that John holds would change everything for them. So, his dad recommends that the "the truth should come little by little" to their people so that it doesn't crash their entire world around them. John agrees, stating that "Perhaps, in the old days, they ate knowledge too fast", which is what helped cause their downfall. This insight is probably the most profoudn of the story, and John realizes that the great knowledge that he possesses must be given in small doses, as the people are ready for it, and can show they can use it wisely.